The present invention relates to methods and apparatus for making tubular composite containers and, more particularly, to methods and apparatus for making such containers by wrapping a liner strip and at least one paperboard strip about an axis and adhering the various strips together.
Food and drink products and other perishable items are often packaged in tubular containers that are sealed at both ends. These tubular containers typically include at least one structural body ply and are formed by wrapping a continuous strip of body ply material around a mandrel of a desired shape to create a tubular structure. The body ply strip may be spirally wound around the mandrel or passed through a series of forming elements so as to be wrapped in a convolute shape around the mandrel. At the downstream end of the mandrel, the tube is cut into discrete lengths and is then fitted with end caps to form the container.
Tubular containers of this type typically include a liner ply on the inner surface of the paperboard body ply. The liner ply prevents liquids such as juice from leaking out of the container and also prevents liquids from entering the container and possibly contaminating the food product contained therein. Preferably, the liner ply is also resistant to the passage of gases, so as to prevent odors of the food product in the container from escaping and to prevent atmospheric air from entering the container and spoiling the food product. Thus, the liner ply provides barrier properties and the body ply provides structural properties.
Conventional liner plies most often include aluminum foil, which has good barrier properties and also has advantageous strength properties. In particular, the liner is wound onto the mandrel prior to the winding of the body ply and must be sufficiently strong and stiff to be independently wound on the mandrel without stretching or wrinkling. In addition, the aluminum foil layer typically includes a kraft paper backing for allowing the foil layer to be adhered to the paperboard body ply. Because of the support provided by the kraft-backed foil layer of the liner, such liners are known as xe2x80x9csupportedxe2x80x9d liners.
Aqueous based adhesives (or xe2x80x9cwet adhesivesxe2x80x9d) are preferred for adhering the liner ply to the body ply because solvent-based adhesives have become disfavored in light of various environmental concerns over their use and disposal. However, it has heretofore been difficult to get the aqueous adhesives to stick to the smooth and impervious surface of the aluminum foil layer. Accordingly, a kraft paper backing has been preadhered to the foil layer so that the liner can be adhered to the paperboard body ply with wet adhesives. Kraft paper also adds additional cost and thickness to the liner.
Typically, a liner having a foil layer includes a polymeric layer on the surface of the foil that faces inward toward the interior of the resulting container. The polymeric layer prevents product in the container from coming into contact with the foil layer, which in some cases can cause a reaction that can corrode the foil and discolor or otherwise adulterate the product. The polymer layer may also improve the abrasion-resistance of the foil. The polymer layer frequently is also a heat-sealable material permitting one edge portion of the liner strip to be heat sealed to an overlying opposite edge portion of the strip.
In the manufacture of tubular composite containers, a strip of liner material is wrapped about a shaping mandrel and is advanced along the mandrel as the tubular composite container is formed. At the line speeds currently being used in the commercial manufacture of such containers, the liner strip is typically advanced at a linear rate of 400 feet per minute or more and is compressed upon the mandrel by the paperboard strips wrapped on top of the liner and by the belt or other device that advances the tubular container along the mandrel. Consequently, there is considerable friction between the mandrel and the liner, which generates heat. This heat can cause softening of the polymer layer of the liner that is in contact with the mandrel, with the result that the liner tends to adhere to the mandrel and is xe2x80x9cscuffedxe2x80x9d as it advances along the mandrel.
To help counteract this scuffing tendency, a lubricant is typically applied to the inner surface of the liner. Additionally, in some cases the mandrel is chilled so that the mandrel temperature is kept sufficiently low that the polymer layer of the liner is less prone to softening and sticking to the mandrel. However, where the polymer film layer performs the function of a heat seal layer, a dilemma of sorts is faced wherein a low mandrel temperature is desirable for reducing scuffing and sticking of the liner on the mandrel, while a higher mandrel temperature is desirable for facilitating the activation of the heat-sealable polymer layer to form a seal.
Where a supported liner is used, this dilemma can be largely avoided by heating the liner, or at least the edge portions that are overlapped to form a seal, to a temperature above the sealing temperature for the polymer layer just prior to wrapping the liner about the mandrel. The relatively large mass of the foil and kraft layers of the liner operates as a heat sink which holds the heat long enough that the liner edge portions can be heat sealed to each other when the liner strip is wrapped on the mandrel.
However, foils are expensive, and so it is desired to provide a container and a method of making such as container which includes an xe2x80x9cunsupportedxe2x80x9d liner having the requisite barrier properties without the aluminum foil layer and kraft layer. When an unsupported liner is used, however, the heat sink function of the foil and kraft layers is eliminated. As a consequence, the relatively thin polymer film liner is difficult to heat because of its low mass which tends to dissipate heat rapidly.
The use of unsupported liners also presents other technical challenges. For example, because of the problems associated with winding an unsupported liner on the mandrel, such as stretching, creasing or other misshaping of the liner, it has not been economically or commercially feasible with conventional winding apparatus and methods to manufacture a container having an unsupported liner ply. Nevertheless, in view of the considerable material and cost savings that unsupported liners offer, it would be highly desirable to provide methods and apparatus for making tubular composite containers with unsupported liners.
Unsupported liners also offer advantages in addition to material and cost savings. More particularly, the elimination of the kraft layer enables straight overlap seams to be used in the liner, rather than the conventional xe2x80x9canaconda foldxe2x80x9d seams traditionally used with kraft-backed liners. In an anaconda fold, the underlying edge of the liner ply is folded back on itself and adhered to the overlying edge. The anaconda fold allows the polymeric layers on the surface of the foil layer to be heat sealed together. Alternatively, a hot melt adhesive can be used to seal the underlying edge of the liner ply to the overlying edge. The edge of the kraft paper thus is not exposed to the interior of the container and thus liquids in the container will not be absorbed by the kraft paper. An example of such a fold is illustrated in U.S. Pat. No. 5,084,284 to McDilda, et al.
Anaconda folds are undesirable, however, because of their increased thickness. The thickness of an anaconda fold seam is equal to three thicknesses of the liner ply. Thus, with relatively thick supported liners, the anaconda fold presents a substantial thickness and poses difficulties when attempting to hermetically seal the ends of the tubular container. Specifically, the ends of the tube are often rolled outwardly after being cut so as to form a rolled circular bead or flange on one or both ends of the tube and then end caps or membranes are applied and usually sealed to the bead with an adhesive sealant, heat sealing, or other technique. However, in the area where the thick anaconda fold seam forms a portion of the edge surface, the end surface of the bead or flange can be substantially non-planar thus forming hill-like and/or valley-like irregularities. Accordingly, an extra amount of adhesive sealant or heat seal material is required in order to fill the discontinuities and hermetically seal the tubular container. The additional application of adhesive sealant or heat seal material is disadvantageous because of the extra sealant that must be used and the increased difficulty in removing the seal by the consumer due to the additional sealant. For example, where a membrane includes a heat seal layer, the entire heat seal layer must be made thicker, even though the increased thickness is actually needed only around the periphery of the membrane where it contacts the bead.
Because of the problems noted above with respect to supported liners, efforts have been made toward developing methods and apparatus for making tubular composite containers having unsupported liners in which the foil and kraft layers are eliminated from the liner. Additionally, there have been sought methods and apparatus for making composite containers having liners formed without anaconda fold seams.
A liner formed entirely of one or more relatively thin polymer layers would be particularly beneficial in that the additional foil and kraft layers would be eliminated along with their attendant costs, and the liner seam would present a relatively slight bump at the curled end of the container so that problems of hermetically sealing closures on the container ends would be substantially reduced. Accordingly, the assignee of the present application has striven toward developing practicable methods and apparatus for making containers with such unsupported film liners. For example, the assignee of the present application has developed methods and apparatus for making composite containers with unsupported liners made of polymer film and without anaconda folds, as disclosed in commonly owned U.S. patent application Ser. No. 08/796,912 entitled xe2x80x9cPolymeric Liner Ply for Tubular Containers and Methods and Apparatus for Manufacturing Samexe2x80x9d filed Feb. 6, 1997, and U.S. Pat. No. 5,829,669 entitled xe2x80x9cTubular Container and Methods and Apparatus for Manufacturing Samexe2x80x9d issued Nov. 3, 1998, the entire disclosures of which are hereby incorporated herein by reference.
The ""912 patent application and ""669 patent disclose composite container-forming methods and apparatus wherein, according to one embodiment, a polymeric liner strip is adhesively joined to a paperboard body-forming strip prior to being wrapped about a shaping mandrel. By xe2x80x9cprelaminatingxe2x80x9d the polymeric liner strip and paperboard strip together, the liner is effectively structurally supported by the paperboard strip so that it is relatively easily advanced to the mandrel without becoming excessively stretched or otherwise misshapen in the process. The polymeric liner strip is offset relative to the paperboard to which it is adhesively joined such that a marginal edge portion of the liner strip extends beyond one edge of the paperboard strip. The laminated paperboard/polymeric strip is wrapped about the mandrel so that the edges of the paperboard strip overlap each other and the marginal edge portions of the polymeric liner overlap each other. One of the marginal edge portions of the liner strip includes a non-aqueous adhesive layer that is heat activatable. Before and/or while the laminated paperboard/polymeric strip is wrapped about the mandrel, the non-aqueous adhesive layer on the marginal edge portion is heated to at least its activation temperature, and the overlapping edges are heat sealed together. Thus, the resulting composite container has a polymeric liner formed without anaconda fold seams.
However, the apparatus used to laminate the body and liner plies together before wrapping the plies onto the mandrel may not be the most advantageous in some applications. For instance, in some cases a separate set of nip rollers is needed to effect the lamination. Additionally, it will be appreciated that both the liner supply and body ply supply rolls must be located on the same side of the mandrel, which can make the placement of the supply rolls and the routing of the plies to the mandrel more complicated than would otherwise be the case if both the liner and body plies did not have to approach the mandrel from the same side. Replacement of the rolls may also be more difficult where the two supply rolls are located close together as they would tend to be in order to make efficient use of space.
In light of the foregoing, it would be highly desirable to provide methods and apparatus capable of making a composite container with an unsupported polymer film liner. Furthermore, it would be desirable to provide methods and apparatus permitting the formation of such a container without prelaminating the body and liner plies. It would also be desirable to provide methods and apparatus allowing either same-side or opposite-side winding of the body and liner plies onto the mandrel so as to increase the flexibility available to the designer and user of the apparatus in locating the body and liner supply rolls.
The above and other objects are met and other advantages are achieved by the present invention, which includes methods and apparatus for making a tubular composite container having a paperboard body strip and an unsupported polymer film liner strip adhered thereto, wherein the unsupported liner strip is wrapped about the mandrel without first being adhered or prelaminated to the paperboard strip.
In accordance with a preferred embodiment of the invention, the liner strip includes a layer of heat-activated sealing material covering at least a first edge portion of the liner strip and a layer of compatible polymer material covering at least an opposite second edge portion of the liner strip. The sealing material has a predetermined sealing temperature at which the material is activated to form a bond with the compatible polymer material. The liner strip is wrapped around the mandrel with the sealing material on the first edge portion of the liner strip facing outward and the compatible polymer material on the second edge portion facing inward toward the mandrel, and the first edge portion is overlapped by the second edge portion to form an overlap joint therebetween having the layer of the sealing material on the first edge portion in contact with the compatible material on the second edge portion. At least the edge portions of the liner strip are preheated at a first heating station such that sealing material reaches an elevated temperature below its predetermined sealing temperature when the edge portions are overlapped to form the overlap joint. Additional heat is then applied locally to the overlap joint of the tubular liner at a second heating station located on the mandrel to further raise the temperature of the sealing material to at least the predetermined sealing temperature and cause the layers of sealing material to bond to the compatible material. Finally, a continuous paperboard body strip is wrapped around the tubular liner on the mandrel and adhered thereto so as to create a tubular container.
In a further preferred embodiment of the invention, preheating of the liner strip is performed by raising the temperature of at least a portion of the mandrel to an elevated temperature and wrapping the liner strip on the mandrel such that the liner strip passes over the elevated-temperature portion of the mandrel prior to reaching the second heating station. In this way, the mandrel temperature can be maintained at a relatively lower temperature than that which would be required were all of the heating for activating the sealing material to be done by heating the mandrel. Thus, scuffing of the liner strip on the mandrel is minimized. The elevated temperature portion of the mandrel can be heated by circulating a heated fluid through the mandrel portion, by disposing resistance-type heating elements within the mandrel portion, or by other means known in the heating art.
In accordance with another preferred embodiment of the invention, the additional heat for activating the sealing material is applied locally to the overlap joint of the liner by heating a portion of the mandrel over which the overlap joint passes to a higher temperature than the elevated-temperature portion of the mandrel. Advantageously, infrared radiation is focused on the overlap joint and penetrates through the liner edge portions such that the mandrel surface underlying the overlap joint is heated to a temperature above the sealing temperature of the sealing material. However, other heating devices can be used in addition to or instead of the infrared radiator, including forced hot air devices directed on the overlap joint, electrical resistance heating elements disposed within the mandrel, or other heating devices known in the heating art.
In order to adhere the liner and paperboard strips together, an adhesive is applied to a surface of the paperboard body strip that confronts the outer surface of the tubular liner. Advantageously, the surface of the liner strip that forms the outer surface of the tubular liner is treated to improve adhesion of the adhesive thereto. The surface treatment in one preferred embodiment comprises corona discharge treatment. Alternatively, flame treatment may be used.
Thin polymer films are stretchable, and accordingly it is preferable to control the tension of the polymer film liner strip to maintain the tension low enough that stretching of the liner strip is substantially avoided. Thus, in a preferred embodiment of the invention, the tension of the liner strip is controlled so that it is less than about 1 pound per inch of width of the liner strip, which permits films having thicknesses of as little as 0.0005 inch to be used for the liner strip.
The invention also encompasses apparatus for making tubular composite containers having unsupported liners. An apparatus according to one preferred embodiment of the invention includes an elongate mandrel having a generally tubular outer surface adapted to permit a polymer film liner strip having a heat-sealing material to be wrapped about and slide over the mandrel. The apparatus also includes a first heater in heat-exchanging relation with a first portion of the mandrel and operable for heating the first portion of the mandrel to an elevated first temperature below a predetermined minimum sealing temperature of the liner heat-sealing material. The apparatus further includes a second heater in heat-exchanging relation with a second portion of the mandrel and operable for heating the second portion of the mandrel to a second temperature above the predetermined minimum sealing temperature. The apparatus also comprises an adhesive applicator for applying adhesive to a surface of a continuous paperboard strip passed through the adhesive applicator. A tube conveyor engages the paperboard strip when it is wrapped about the mandrel and adhered to the liner strip on the mandrel. The tube conveyor advances the composite tube lengthwise along the mandrel as the tube is being formed such that the liner strip is drawn over the first portion of the mandrel and then over the second portion thereof. The first and second heaters at the first and second portions of the mandrel progressively heat at least opposite edge portions of the liner strip to the predetermined minimum sealing temperature for heat-sealing one of the edge portions to the other at an overlap formed therebetween.
Advantageously, the first heater comprises a fluid passage formed within the first portion of the mandrel for circulating a heated fluid therethrough to heat the first portion to the first temperature. The second heater preferably comprises an infrared radiator positioned to direct infrared radiation on an overlap joint of a liner strip on the mandrel. Particularly where a thin unsupported polymer liner is moving over the mandrel at high speeds of up to 400 feet per minute, the liner has very little time to absorb heat. Thus, to heat the liner overlap joint to the seal point, the mandrel is used as a heat sink so as to absorb the infrared radiation. The unsupported polymer liner without foil layer is substantially transparent to infrared radiation, and accordingly the infrared radiation passes through the liner strip overlap joint and heats the surface of the mandrel underlying the overlap joint to the second temperature. The mandrel surface in turn heats the sealing material on the liner to cause the edge portions of the liner to be sealed together. If the polymer film liner were to be heated to the seal point without the use of the mandrel as a heat sink, the film would tend to melt or shrink.
The apparatus preferably also includes a surface treatment unit that modifies the surface of the liner strip to improve wetting and adhesion of adhesive to the liner strip. The surface treatment unit preferably comprises a corona discharge unit. However, other devices can be used instead, such as a flame treatment unit as known in the art.
In a preferred embodiment of the invention, the apparatus also includes a tension control unit for controlling the tension of the liner strip. The tension control unit advantageously is of the type having a web accumulator that accumulates a length of liner strip material from which the liner strip is drawn at substantially constant tension. However, other types of tension control devices known in the art may be used. Preferably, the tension control unit is operable to maintain the tension of the liner strip less than about 1 pound per inch of width of the liner strip as the liner strip is wrapped about the mandrel. Accordingly, even very thin unsupported polymer film liner strips may be used without substantial problems of stretching or other misshaping of the strip.
The invention also encompasses tubular containers having unsupported polymer film liners. In accordance with a preferred embodiment of the invention, a tubular container comprises at least one body ply formed of paperboard and wrapped into a tubular shape having an inner surface, the body ply defining a predetermined circumferential length when in an unwrapped, relaxed, and flat state, and a polymeric liner ply wrapped into a tubular shape and having an outer surface adhered to the inner surface of the body ply, the liner ply defining a predetermined circumferential length when in an unwrapped, relaxed, and flat state. The circumferential length of the liner ply is less than that of the body ply such that the liner ply is substantially uncompressed circumferentially. Accordingly, the liner can be made to lie smoothly against the adjacent body ply so as to present an attractive appearance.